Electrophotographic copiers are well known in the art. In copiers of this type, a photoconductive imaging surface, such as a selenium layer supported by a conductive cylindrical substrate, is first provided with a uniform electrostatic charge, typically by moving the surface at a uniform velocity past a charge corona. The imaging surface, which in the case of selenium now bears a positive potential of about 1,000 volts, is exposed to an optical image of an original to selectively discharge the surface in a pattern forming an electrostatic latent image. In the case of a typical original bearing dark print on a light background, this latent image consists of substantially undischarged "print" portions, corresponding to the graphic matter on the original, amidst a "background" portion that has been substantially discharged by exposure to light. The latent-image-bearing surface is then developed by oppositely charged pigmented toner particles, which deposit on the print portions of the latent image in a pattern corresponding to that of the original. In liquid-developer copiers, these particles are suspended in an insulating carrier liquid which is applied to the photoconductive surface.
One of the problems inherent in electrophotographic copiers has been the unwanted deposition of toner particles onto background portions of the latent image, which retain a background potential of about 100 volts even after exposure to light. One solution to this problem, as shown in Schaefer et al U.S. Pat. No. 3,892,481, Kuroishi et al U.S. Pat. No. 4,021,111, and Miyakawa et al U.S. Pat. No. 4,050,806, has been the disposition of a developing electrode in the developing station closely adjacent to the latent-image-bearing surface. The developing electrode is supplied with a biasing potential slightly above the residual potential of the background portions of the latent image, but well below the potential of the undischarged print portions of the image. Developer liquid is supplied to the region between the developing electrode and the photoconductive surface.
In such an arrangement, suspended toner particles in regions adjacent to the background portions are attracted to the developing electrode, which is more positive than the adjacent background portions of the latent image. At the same time, toner particles adjacent to the undischarged print portions of the latent image are attracted to these portions of the image, which are at a much higher potential than the developing electrode. In this manner, toner deposition on background portions of the image can be reduced or eliminated.
Although electrophotographic copiers of the type described above have proven successful in eliminating the problem of background staining, there remain areas for further improvement. Thus, while regulating the biasing potential adequately controls the density of the background portion of the developed image, it has little effect on the density of the print portions of the image.
It is also known in the art to use an electrometer to control the rate at which a photoconductive surface is charged. Such systems are disclosed, for example, in Weber U.S. Pat. No. 4,431,302, Fantozzi U.S. Pat. No. 4,341,461, and Tabuchi U.S. Pat. No. 4,432,634. Each of these systems, however, has one or more drawbacks. Thus, the Weber system is concerned with the control of charge level only, and would require an entirely independent system to control the density of the background portions of the developed image. Tabuchi is concerned primarily with maintaining a constant difference between the charge potential and the biasing potential (column 4, lines 7 to 18; Claim 1, column 8, lines 8 to 14). Tabuchi does not suggest, nor would the disclosed system be readily adaptable to, independent control of the charge potential and the biasing potential. Likewise, in Fantozzi, substantially independent systems are used for control of charging and biasing potential, increasing the overall cost and complexity of the system. Moreover, in all three of these disclosures, the electrometer operates through an air gap, creating inevitable inaccuracies of measurement.
Still other problems inherent in systems of the prior art relate to the bias control system itself. As disclosed in the above-identified Schaefer et al and Kuroishi et al patents, it is known in the art to supply the development electrode with an opposite-polarity cleaning potential between successive copies. This cleaning potential repels accumulated toner particles from the development electrode onto the photoconductive surface, from which the toner particles are eventually removed at a cleaning station. In this manner, one avoids the buildup of toner particles on the development electrode, which would impair operation. Such a cleaning cycle, however, imposes an upper limit on the copy rate. Thus, if the development electrode extends a distance L1 along the path of the photoconductor, and the photoconductor itself moves a distance L2 during the application of a cleaning potential to the development electrode, the total extent of the photoconductor surface used to remove toner particles from the development electrode is L1+L2. This extent of the photoconductive surface is unavailable for the formation of a latent image of a successive original, and necessitates a minimum interval between copies.